1. Field of the Invention
This invention relates to fluid pumps and, more particularly, to an Archimedean screw-type pump.
2. Background Art
It is known to use an Archimedean screw-type pump to convey fluids and, in one application, to hoist water from a reservoir in a waste treatment facility. The pump has a cylindrical casing defining a fluid conveying space within which a coaxially arranged, rotatable, broad-threaded screw or spirally bent tube is fixed. The casing has open inlet and outlet ends, with the latter situated above the maximum anticipated height of the water to be pumped. In operation, the pump inlet is dipped into the liquid to be elevated with the pump casing inclined at less than 90.degree. to horizontal. As the screw and tube are rotated, the screw flights hoist water progressively upwardly and towards the outlet where it spills over and is suitably collected.
An exemplary prior art lift is shown in U.S. Pat. No. 3,077,932, to Gehrke. Gehrke discloses a spiral lift for use in oil recovery operations. One problem inherent in the Gehrke structure is that with the pump inlet submerged there is a tendency of the screw to cavitate i.e. to evacuate the space above the level of the oil supply and create air pockets in the oil that is being conveyed. This results in reduction in pump delivery capacity and potentially a backflow of oil from the upper screw flights.
One solution to the cavitation problem is proposed in U.S. Pat. No. 1,085,949, to Snyder. In Snyder, a cylindrical core is provided substantially along the entire length of a hoisting screw. The core has a hollow center which communicates with the atmosphere and a helical vent to equalize pressure in the pump by establishing communication between the inside of the core and the spaces between adjacent screw turns. Development of low pressure areas between any of the screw turns is thereby alleged to be prevented by Snyder.
There are inherent drawbacks with the Snyder structure. First, the slotted core has no structural utility other than to establish a communication path between the atmosphere and the air spaces between the screw turns. The core takes up space on the surface of the screw flights that otherwise would be useable to convey fluid. Further, the continuous spiral slot compromises the integrity of the core and the attached screw. Still further, the requirement of the vent throughout the entire length of the core complicates manufacture of the Snyder screw pump.